testthis: Utils and 'RStudio' Addins to Make Testing Even More Fun

Utility functions and 'RStudio' addins to ease the life of people using
'testthat', 'devtools' and 'usethis' in their package development workflow.
Hotkeyable addins are provided for such common tasks as switching between a
source file and an associated test file, or running unit tests in a single
file. 'testthis' also provides utility function to manage and run tests in
subdirectories of the test/testthat directory.